JP6434515B2 - Radiation system and lithographic apparatus - Google Patents

Description

[Cross-reference of related applications]
[0001] This application claims the benefit of US Provisional Application No. 61 / 870,128, filed Aug. 26, 2013, which is hereby incorporated by reference in its entirety.

  The present invention relates to a radiation source and a lithographic apparatus.

  [0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that case, a patterning device, also referred to as a mask or a reticle, may be used to generate a circuit pattern formed on an individual layer of the IC. This pattern can be transferred onto a target portion (eg including part of, one, or more dies) on a substrate (eg a silicon wafer). Usually, the pattern is transferred by imaging on a radiation-sensitive material (resist) layer provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

  [0004] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and / or structures. However, as the dimensions of features created using lithography become smaller, lithography has become a more important factor to enable the manufacture of small ICs or other devices and / or structures.

ここで、λは、使用される放射の波長であり、ＮＡは、パターンを印刷するために使用される投影システムの開口数であり、ｋ_１は、レイリー定数とも呼ばれるプロセス依存調整係数であり、ＣＤは、印刷されたフィーチャのフィーチャサイズ（又はクリティカルディメンジョン）である。式（１）から、フィーチャの最小印刷可能サイズは、露光波長λを短くすること、開口数ＮＡを大きくすること、あるいはｋ_１の値を小さくすること、の３つの方法によって縮小することができると言える。 [0005] A theoretical estimate of the limit of pattern printing is obtained by the Rayleigh criterion for resolution shown in equation (1).

Where λ is the wavelength of radiation used, NA is the numerical aperture of the projection system used to print the pattern, k ₁ is a process dependent adjustment factor, also called the Rayleigh constant, CD is the feature size (or critical dimension) of the printed feature. From equation (1), the minimum printable size of a feature can be reduced by three methods: shortening the exposure wavelength λ, increasing the numerical aperture NA, or decreasing the value of k _1. It can be said.

  [0006] It has been proposed to use an extreme ultraviolet (EUV) radiation source to shorten the exposure wavelength and thus reduce the minimum printable size. EUV radiation is electromagnetic radiation having a wavelength in the range of 5-20 nm, for example in the range of 13-14 nm, for example in the range of 5-10 nm, such as 6.7 nm or 6.8 nm. Possible radiation sources include, for example, laser-produced plasma sources, discharge plasma sources, or radiation sources based on synchrotron radiation provided by electron storage rings.

  [0007] EUV radiation can be generated using a plasma. A radiation system that generates EUV radiation may include a laser source that excites fuel and provides a plasma, and a source collector module that contains the plasma. The plasma can be generated, for example, by directing a laser beam to a fuel, such as a drop of a suitable material (eg, tin), a suitable gas flow or vapor flow (Xe gas, Li vapor, etc.). The resulting plasma emits output radiation that is collected using a radiation collector, eg, EUV radiation. The radiation collector can be a mirror normal incidence radiation collector that receives the radiation and focuses the radiation into a beam. The source collector module can include a closed structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is commonly referred to as a laser produced plasma (LPP) source.

[0008] Another known method of generating EUV radiation is known as dual laser pulsing (DLP). In the DLP method, for example, by preheating the droplets with a yttrium-aluminum-garnet (Nd: YAG) laser doped with neodymium, the droplets (eg, tin droplets) are decomposed into vapor and small particles, which are then it is heated to very high temperatures by the CO ₂ laser.

  [0009] In known methods such as the LPP method and the DLP method, a droplet stream is generated. The droplets can be generated as a continuous stream or a pulse.

  [0010] For example, in one known method used, particularly with respect to the LPP method, a heated container is filled with molten tin that travels from the container to the capillary via a filter and a piezo actuator. A continuous jet, whose speed is adjusted by a piezo actuator, exits the end of the capillary. During flight, the jet breaks up into smaller droplets, and due to the adjusted speed, these smaller droplets become larger, more closely spaced, larger droplets.

[0011] The laser beam that preheats the droplets and breaks them down into vapor and small particles may deviate slightly from the droplets that the laser beam preheats. Such small deviations may cause further misalignment when CO ₂ laser heats a vapor and small particles to a very high temperature. Such further deviation can adversely affect the amount of EUV radiation emitted by the resulting plasma.

  [0012] According to one aspect of the invention, a radiation source is provided that includes a nozzle configured to direct a fuel droplet stream along a droplet path toward a plasma formation location. The radiation source has a Gaussian intensity distribution, is configured to receive a Gaussian radiation beam having a predetermined wavelength and propagating along a predetermined trajectory. The radiation source is further configured to focus the radiation beam onto the fuel droplets at the plasma formation location. The radiation source includes a phase plate structure comprising one or more phase plates. The phase plate structure has a first zone and a second zone. These zones are arranged such that radiation having a predetermined wavelength passing through the first zone and radiation having a predetermined wavelength passing through the second zone propagate along respective optical paths having different optical path lengths. . The radiation passing through the first zone and the radiation passing through the second zone when the radiation passing through the first zone and the radiation passing through the second zone impinge on one of the fuel droplets at the plasma forming position. The optical path length difference between them is an odd multiple of half the predetermined wavelength.

  The effect of this aspect is that the profile of the radiation beam can be adjusted to be flatter and wider at the plasma formation position.

  [0014] By increasing the tolerance of the alignment requirement of the radiation beam with respect to the droplet, the problem that slight deviations adversely affect the amount of EUV radiation emitted can be solved.

  [0015] The radiation passing through the first zone and the radiation passing through the second zone may be different parts of the Gaussian radiation beam. Radiation passing through the first zone may include at least the top of the intensity distribution. Radiation passing through the second zone may be located away from the top of the intensity distribution, which allows at least a portion of the end of the Gaussian curve to be out of phase with respect to the top of the intensity distribution. obtain.

  [0016] According to one aspect of the present invention, the phase plate structure includes two phase plates, and at least one of the phase plates includes at least a first region and a second region, and passes through the first region. Radiation having a predetermined wavelength and radiation having a predetermined wavelength passing through the second region propagate along respective optical paths, and an optical path between the radiation passing through the first region and the radiation passing through the second region. The difference in length is an odd multiple of half the predetermined wavelength at a position on the trajectory of the radiation beam downstream relative to the phase plate.

  [0017] According to one aspect of the present invention, the phase plate structure includes two phase plates, and at least two of the phase plates include at least a first region and a second region, and pass through the first region. Radiation having a predetermined wavelength and radiation having a predetermined wavelength passing through the second region propagate along respective optical paths, and an optical path between the radiation passing through the first region and the radiation passing through the second region. The difference in length is an odd multiple of half the predetermined wavelength at a position on the trajectory of the radiation beam downstream relative to the phase plate.

  [0018] The one or more phase plates may be formed from ZnSe and / or ZnS.

  [0019] Further features and advantages, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Based on the teachings herein, additional embodiments will be apparent to persons skilled in the art.

  [0020] Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings. In these drawings, the same reference signs indicate corresponding or functionally similar parts.

[0021] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention; [0022] Figure 2 depicts the lithographic apparatus of Figure 1 in more detail, including a source collector module having a normal incidence mirror. [0023] FIG. 3 schematically illustrates a beam delivery system of the source collector module shown in FIG. [0024] FIG. 5 schematically shows a phase plate structure of the beam delivery system of FIG.

  [0025] References to "one embodiment," "one embodiment," "exemplary embodiment," and the like in the specification may refer to specific features, structures, or characteristics of the described embodiment. Although not shown, all embodiments may not include that particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Also, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is explicitly explained that such feature, structure, or characteristic is brought about in the context of other embodiments. It is understood that this is within the knowledge of those skilled in the art.

  FIG. 1 schematically depicts a lithographic apparatus 100 according to one embodiment. The lithographic apparatus includes an EUV radiation source. The lithographic apparatus is constructed to support an illumination system (illuminator) IL configured to condition a radiation beam B (eg EUV radiation) and a patterning device (eg mask or reticle) MA and patterning A support structure (eg, a mask table) MT coupled to a first positioner PM configured to accurately position the device, and a substrate W (eg, a resist-coated wafer); A substrate table (eg, a wafer table) WT coupled to a second positioner PW configured to accurately position a pattern applied to the radiation beam B by the patterning device MA, and a target portion C (eg, (Including one or more dies) Projection system (e.g., reflective projection system) and a PS.

  [0027] Illumination systems include refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, to induce, shape or control radiation Various types of optical components can be included.

  [0028] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as whether or not the patterning device is held in a vacuum environment. The support structure can hold the patterning device using mechanical, vacuum, electrostatic or other clamping techniques. The support structure may be, for example, a frame or table that may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

  [0029] The term "patterning device" should be interpreted broadly to refer to any device that can be used to provide a pattern in a cross section of a radiation beam so as to create a pattern in a target portion of a substrate. . The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

  [0030] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. One example of a programmable mirror array uses a matrix array of small mirrors, and each small mirror can be individually tilted to reflect the incoming radiation beam in various directions. The tilted mirror patterns the radiation beam reflected by the mirror matrix.

  [0031] The projection system may be refractive, reflective, magnetic, electromagnetic, electrostatic, or suitable for the exposure radiation being used, or other factors such as the use of vacuum, as well as the illumination system. Various types of optical components can be included, such as other types of optical components, or any combination thereof. For EUV radiation, it may be desirable to use a vacuum because other gases may absorb too much radiation. Thus, a vacuum wall and vacuum pump may be used to provide a vacuum environment for the entire beam path.

  [0032] As shown herein, the lithographic apparatus is of a reflective type (eg employing a reflective mask).

  [0033] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and / or two or more mask tables). In such “multi-stage” machines, additional tables can be used in parallel, or one or more tables can be used for exposure while a preliminary process is performed on one or more tables. It can also be used.

  [0034] Referring to FIG. 1, the illuminator IL receives an extreme ultraviolet (EUV) radiation beam from a source collector module SO. Methods for generating EUV radiation include converting a material into a plasma state having at least one element, such as xenon, lithium or tin, and having one or more emission lines in the EUV range. However, it is not necessarily limited to this. One such method, often referred to as laser generated plasma (“LPP”), involves irradiating a laser beam with a fuel, such as a droplet of a material having an element that emits a desired emission line. Thus, a desired plasma can be generated. The source collector module SO may be part of an EUV radiation source that includes a laser (not shown in FIG. 1) to provide a laser beam that excites the fuel. The resulting plasma emits output radiation, such as EUV radiation, which is collected using a radiation collector disposed in the source collector module SO.

[0035] For example, if using a CO ₂ laser provides a laser beam for fuel excitation laser and the source collector module may be a separate component. In such cases, the radiation beam is sent from the laser to the source collector module using, for example, a beam delivery system that includes a suitable guide mirror and / or beam expander. The laser and fuel supply can be considered to constitute an EUV radiation source.

  [0036] The illuminator IL may include an adjuster for adjusting the angular intensity distribution of the radiation beam. In general, at least the outer and / or inner radius ranges (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the illuminator pupil plane can be adjusted. In addition, the illuminator IL may include various other components such as a facetted field and a pupil mirror device. By adjusting the radiation beam using an illuminator, a desired uniformity and intensity distribution can be provided in the cross section of the radiation beam.

  [0037] The radiation beam B is incident on the patterning device (eg, mask) MA, which is held on the support structure (eg, mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (eg mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto the target portion C of the substrate W. Using the second positioner PW and the position sensing system PS2 (eg using an interferometer device, linear encoder or capacitive sensor), for example, to position various target portions C in the path of the radiation beam B In addition, the substrate table WT can be moved accurately. Similarly, the first positioner PM and another position sensing system PS1 can be used to accurately position the patterning device (eg mask) MA with respect to the path of the radiation beam B. Patterning device (eg mask) MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

  [0038] The example apparatus can be used in at least one of the modes described below.

  [0039] In step mode, the entire pattern applied to the radiation beam is projected onto the target portion C at a time (ie, simply while maintaining the support structure (eg mask table) MT and substrate table WT essentially stationary). One static exposure). Thereafter, the substrate table WT is moved in the X and / or Y direction so that another target portion C can be exposed.

  [0040] 2. In scan mode, the support structure (eg, mask table) MT and substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (ie, a single dynamic exposure). . The speed and direction of the substrate table WT relative to the support structure (eg mask table) MT can be determined by the (reduction) magnification factor and image reversal characteristics of the projection system PS.

  [0041] 3. In another mode, with the programmable patterning device held, the support structure (eg, mask table) MT is kept essentially stationary and the substrate table WT is moved or scanned while being attached to the radiation beam. The pattern being projected is projected onto the target portion C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device is also required after every movement of the substrate table WT or between successive radiation pulses during a scan. Will be updated accordingly. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as described above.

  [0042] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

  FIG. 2 shows the lithographic apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged so that a vacuum environment can be maintained within the closed structure 220 of the source collector module.

  [0044] The laser source LA supplies xenon (Xe), tin (supplied) from a fuel droplet flow generator 200 having a nozzle configured to guide the droplet flow along the path toward the plasma formation position 211. It is arranged to deposit laser energy via a laser beam 205 in a fuel such as Sn) or lithium (Li). As a result, a highly ionized plasma 210 is created at the plasma formation position 211 having an electron temperature of several tens of electron volts (eV). The energy radiation generated during ion de-excitation and recombination is emitted from the plasma and collected and focused by a near normal incidence radiation collector CO. Both the laser system LA and the droplet flow generator 200 can be considered to constitute an EUV radiation source. An EUV radiation source is sometimes referred to as a laser produced plasma (LPP) source.

  [0045] The radiation reflected by the radiation collector CO is focused on the virtual radiation source point IF. The virtual radiation source point IF is usually referred to as an intermediate focus, and the source collector module SO is arranged such that the intermediate focus IF is located at or near the opening 221 of the closing structure 220. The virtual radiation source point IF is an image of the radiation emission plasma 210.

  [0046] Subsequently, the radiation traverses the illumination system IL. The illumination system IL provides a desired angular distribution for the radiation beam 21 at the patterning device MA and a facet field mirror device 22 and a facet pupil arranged to provide the desired uniformity of radiation intensity at the patterning device MA. A mirror device 24 may be included. When the radiation beam 21 is reflected at the patterning device MA held on the support structure MT, a patterned beam 26 is formed which passes through the reflecting elements 28, 30 to the wafer stage or substrate table WT. The image is formed on the substrate W held by the projection system PS.

  [0047] The laser system LA may be used to preheat the fuel. This is illustrated in FIG. FIG. 3 schematically shows the laser system LA. The laser source LA comprises two laser sources 301a, 301b constructed and arranged to produce pulsed radiation beams 303a, 303b. The main pulse laser source 301a may be configured to generate radiation having a wavelength of 10.59 μm, and the prepulse laser source 301b may be configured to generate radiation having a wavelength of 10.23 μm. The mirror 302a and the beam splitter 302b reflect radiation having a wavelength of 10.23 μm as shown in FIG.

  [0048] The embodiment of FIG. 3 is configured such that, in use, the laser source 301b is initially activated to produce a pulse, eg, after 1 μs, the laser source 301a is activated to produce a pulse.

  [0049] The laser system LA includes a beam delivery system 305. The beam delivery system 305 includes reflectors 307 and 309 and beam splitters 311, 313 and 315. The reflectors 307 and 309 and the beam splitters 311, 313 and 315 are configured so that the radiation beam propagates along a predetermined main locus 317 and a predetermined prepulse locus 319. The beam splitter 311 is configured to reflect radiation having a wavelength of about 10.59 μm and transmit radiation having a wavelength of about 10.23 μm. Thus, the pulses generated by the laser source 301a propagate along the trajectory 317, and the pulses generated by the laser source 301b propagate along the trajectory 319. Both trajectories 317 and 319 pass through the focus unit 320, which focuses the radiation beam to the plasma generation position 211 and collides with one of the droplets 322.

  [0050] The beam delivery system 305 of FIG. 3 includes a phase plate structure 321, which is shown in detail in FIG. The phase plate structure 321 includes a first phase plate 323 and a second phase plate 325. Each of the phase plates 323 and 325 includes first regions 327 and 329 and second regions 331 and 333. The phase plates 323 and 325 are positioned and oriented so that a portion of the laser beam 303 propagating along a predetermined prepulse trajectory 319 propagates through both the first phase plate 323 and the second phase plate 325. The

  Each of the phase plates 323 and 325 has a difference in optical path length between the radiation from the radiation beam 303 passing through the first regions 327 and 329 and the radiation from the radiation beam 303 passing through the second regions 331 and 333. Constructed and arranged as follows. This difference may be an odd multiple of half the wavelength of the radiation beam 303. Such a wavelength may be the wavelength of radiation of the prepulse laser source 301b, in this embodiment a wavelength of about 10.23 μm.

  [0052] In FIG. 4, a cross-section 335 of the radiation beam 303 is shown. A portion of the radiation beam passes through the first zone 337 and another portion of the radiation beam passes through the second zone 339. Thereby, when the radiation of the radiation beam 303 is in-phase upstream with respect to the phase plate structure 321, the radiation transmitted through the first zone 337 is out of phase with the radiation transmitted through the second zone 339. Those skilled in the art will readily understand that a phase shift is provided. The size of the first zone 337 is determined by the positions of the first phase plate 323 and the second phase plate 325, and one or both of the positions of these phase plates is determined by the actuator system 341 shown in FIG.

  Returning to FIG. 3, the final focus metrology unit 343 provides data 345 regarding the intensity profile and wavefront of the radiation beam 303. Data processing system 347 calculates a beam profile near the focus of the combination of focus unit 320 and beam splitters 313,315. This may be the case when the radiation beam has passed through the phase structure 321. The data processing system 347 operates an actuator system 341 that positions the phase plates 323 and 325 with respect to a portion of the radiation beam that passes along the trajectory 319.

  The positions of the phase plates 323 and 325 operate so that the cross section of the beam at the plasma formation position 211 propagated along the prepulse locus 319 has a flatter profile rather than a Gaussian shape.

  [0055] In operation, the pre-pulse laser source 302b initially generates a pulse. This pulse propagates through the phase plate structure 321 along the locus 319. This flattens the wavefront of the pulse as described above. The pulse is applied to a droplet 322 that vaporizes into a pancake-shaped cloud. Next, the main pulse laser source 302a is activated to generate a pulse that propagates along the trajectory 317, and collides with the cloud to generate EUV emission plasma.

[0056] It should be understood that many variations and modifications may be made without departing from the invention. Instead of above prepulse laser source which may be a CO ₂ laser, for example, yttrium neodymium to generate radiation doped with a wavelength of about 1.064 .mu.m - Aluminum - Garnet (Nd: YAG) laser can be used.

  [0057] In general, there may be more elements in the illumination system IL and projection system PS than shown. In addition, there may be more mirrors than shown, for example 1 to 6 additional reflective elements may be present in the projection system PS than shown in FIG. One feature of the notable source collector module SO is that it is angled with respect to the laser source, which is supplied to the laser source LA in order to avoid fuel droplets colliding with the collector CO. This means that the fuel droplet stream needs to be substantially horizontal.

  [0058] Although specific reference is made herein to the use of a lithographic apparatus in IC manufacturing, a lithographic apparatus described herein is disclosed in an integrated optical system, a guidance pattern and a detection pattern for a magnetic domain memory, It should be understood that other applications such as the manufacture of flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, LEDs, solar cells, photonic devices, etc. may be used. As will be appreciated by those skilled in the art, in such other applications, the terms “wafer” or “die” as used herein are all more general “substrate” or “target”, respectively. It may be considered synonymous with the term “part”. The substrate described herein may be used, for example, before or after exposure, such as a track (usually a tool for applying a resist layer to the substrate and developing the exposed resist), a metrology tool, and / or an inspection tool. May be processed. Where applicable, the disclosure herein may be applied to substrate processing tools such as those described above and other substrate processing tools. Further, since the substrate may be processed multiple times, for example, to make a multi-layer IC, the term substrate as used herein may refer to a substrate that already contains multiple processing layers.

  [0059] The term "lens" can refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components, depending on the context. .

  [0060] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative rather than limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims (11)

A nozzle for directing a fuel droplet stream along a droplet path toward a plasma formation position;
A radiation source having a Gaussian intensity distribution, having a predetermined wavelength, receiving a Gaussian radiation beam propagating along a predetermined trajectory, and focusing the radiation beam on a fuel droplet at the plasma formation position;
A phase plate structure comprising at least two phase plates, comprising a first zone and a second zone, wherein the zone passes through the first zone through the radiation having the predetermined wavelength and through the second zone. A phase plate structure, wherein the radiation having the predetermined wavelength is arranged to propagate along respective optical paths having different optical path lengths, and
The radiation passing through the first zone when the radiation passing through the first zone and the radiation passing through the second zone impinge on one of the fuel droplets at the plasma formation location; The optical path length difference from the radiation passing through a second zone is an odd multiple of half the predetermined wavelength;
At least two of the at least two phase plates comprise at least a first region and a second region, the radiation having the predetermined wavelength passing through the first region and the predetermined region passing through the second region. Radiation having a wavelength propagates along each optical path, and the difference in optical path length between the radiation passing through the first region and the radiation passing through the second region is relative to the phase plate. An odd multiple of half the predetermined wavelength at a position on the trajectory of the radiation beam downstream
The at least two phase plates are arranged in a direction orthogonal to the radiation beam so that the size and / or position of the first zone and / or the size and / or position of the second zone can be adjusted. Moveable relative to each other,
Radiation system.   The radiation system according to claim 1, wherein the radiation passing through the first zone and the radiation passing through the second zone are different portions of the Gaussian radiation beam.   The radiation system according to claim 2, wherein the radiation passing through the first zone includes at least a top portion of the intensity distribution.   4. A radiation system according to claim 2 or 3, wherein the radiation passing through the second zone is located away from the top of the intensity distribution. Wherein the predetermined wavelength is 9Myuemu～11myuemu, radiation system according to any one of claims 1-4. A focus metrology unit that provides data on the intensity profile and wavefront of the radiation beam after passing through the phase plate structure;
A data processing system for calculating a beam profile at the plasma formation position based on the data;
An actuator system for positioning the at least two phase plates based on the beam profile;
The radiation system according to any one of claims 1 to 5 , further comprising: The radiation system is intended to generate extreme ultraviolet radiation, CO ₂ laser source or yttrium - aluminum - comprises further a laser source such as a garnet laser source, the radiation system according to any one of claims 1-6. Providing a pre-pulse that collides with the fuel droplet at the plasma formation position and a subsequent main pulse that collides with the fuel droplet at the plasma formation position after the pre-pulse has collided with the fuel droplet; The radiation system of claim 7 , wherein the radiation system produces an extreme ultraviolet radiation produced plasma. 9. The radiation system of claim 8 , wherein the radiation system is configured such that, in use, the laser source generates the pre-pulse and the subsequent main pulse. The radiation system is configured such that, in use, the laser source generates the prepulse, the radiation system comprises a further laser source, and the radiation system is in use, the additional laser source generates the main pulse. The radiation system of claim 8 , configured to: Lithographic apparatus comprising a radiation system according to any one of claims 1-10.

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